We have recently shown that prolactin administration to fetal rabbits accelerated lung maturation, manifested both by increased synthesis of dipalmitoyllecithin and increased lung compliance. We propose to study the role of prolactin in lung maturation by investigating: 1. the effective physiological dose; 2. interaction with other hormones; 3. the temporal relationship of endogenous prolactin secretion to cortisol and thyroxine secretion; 4. the site of action. We have evidence to suggest that prolactin might be a natural trigger of surfactant synthesis. By clarifying the above questions, prolactin could emerge as a useful modality in the prevention or treatment of respiratory distress syndrome of the newborn.